I. Field of the Invention
This invention relates to the use of sacrificial agents in fly ash concrete and similar cementitious mixtures, and to the resulting mixtures and compositions. More particularly, the invention relates to sacrificial agents that reduce or eliminate detrimental effects of fly ash on the air entrainment properties of fresh concrete and similar mixtures.
II. Background Art
The partial replacement of portland cement by fly ash is growing rapidly, driven simultaneously by more demanding performance specifications on the properties of concrete and by increasing environmental pressures to reduce portland cement consumption. Fly ash can impart many beneficial properties to concrete such as improved rheology, reduced permeability and increased later-age strength; however, it also may have a negative influence on bleed characteristics, setting time and early strength development. Many of these issues can be managed by adjusting mixture proportions and materials, and by altering concrete placement and finishing practices. However, other challenging problems encountered when using certain fly ash are not always easily resolved. The most important difficulties experienced when using fly ash are most often related to air entrainment in concrete.
Air entrained concrete has been utilized in the United States since the 1930's. Air is purposely entrained in concrete, mortars and grouts as a protective measure against expansive forces that can develop in the cement paste associated with an increase in volume resulting from water freezing and converting to ice. Adequately distributed microscopic air voids provide a means for relieving internal pressures and ensuring concrete durability and long term performance in freezing and thawing environments. Air volumes (volume fraction) sufficient to provide protective air void systems are commonly specified by Building Codes and Standard Design Practices for concrete which may be exposed to freezing and thawing environments. Entrained air is to be distinguished from entrapped air (air that may develop in concrete systems as a result of mixing or the additions of certain chemicals). Entrained air provides an air void system capable of protecting against freeze/thaw cycles, while entrapped air provide no protection against such phenomena.
Air is also often purposely entrained in concrete and other cementitious systems because of the properties it can impart to the fresh mixtures. These can include: improved fluidity, cohesiveness, improved workability and reduce bleeding.
The air void systems are generated in concrete, mortar, or paste mixtures by introducing air entrainment admixtures (referred to as air entrainment agents or air-entraining agents) which are a class of specialty surfactants. When using fly ash, the difficulties in producing air-entrained concrete are related to the disruptive influence that some fly ashes have on the generation of sufficient air volumes and adequate air void systems. The primary influencing factor is the occurrence of residual carbon, or carbonaceous materials (hereafter designated as fly ash-carbon), which can be detected as a discrete phase in the fly ash, or can be intimately bound to the fly ash particles. Detrimental effects on air entrainment by other fly ash components may also occur, and indeed air entrainment problems are sometimes encountered with fly ash containing very low amounts of residual carbon.
Fly ash-carbon, a residue of incomplete coal or other hydrocarbon combustion, is in many ways similar to an ‘activated carbon’; as typical of the latter, fly ash-carbon can adsorb organic molecules in aqueous environments. In cement paste containing organic chemical admixtures, the fly ash-carbon can thus adsorb part of the admixture, interfering with the function and performance of the admixture. The consequences of this adsorption process are found to be particularly troublesome with air entrainment admixtures (air entrainment agents) which are commonly used in only very low dosages. In the presence of significant carbon contents (e.g. >2 wt %), or in the presence of low contents of highly reactive carbon or other detrimental fly ash components, the air entrainment agents may be adsorbed, interfering with the air void formation and stability; this leads to tremendous complications in obtaining and maintaining specified concrete air contents.
To minimize concrete air entrainment problems, ASTM guidelines have limited the fly ash carbon content to less than 6 wt %; other institutions such as AASHTO and State departments of transportation have more stringent limitations. Industry experience indicates that, in the case of highly active carbon (for example, high specific surface area), major interferences and problems can still be encountered, even with carbon contents lower than 1 wt %.
Furthermore, recent studies indicate that, while fly ash carbon content, as measured by loss on ignition (LOI) values, provides a primary indicator of fly ash behaviour with respect to air entrainment, it does not reliably predict the impact that a fly ash will have on air entrainment in concrete. Therefore, there currently exist no means, suitable for field quality control, capable of reliably predicting the influence that a particular fly ash sample will have on air entrainment, relative to another fly ash sample with differing LOI's, sources, or chemistries. In practice, the inability to predict fly ash behaviour translates into erratic concrete air contents, which is currently the most important problem in fly ash-containing concrete.
Variations in fly ash performance are important, not only because of their potential impact on air entrainment and resistance to freeze thaw conditions, but also because of their effects related to concrete strength. Just as concrete is designed according to Building Standards for a particular environment, specifications are also provided for physical performance requirements; a common performance requirement being compressive strength. An increase in entrained air content can result in a reduction in compressive strength of 3-6% for each additional percentage of entrained air. Obviously, variations in fly ash-carbon, which would lead to erratic variations in air contents, can have serious negative consequences on the concrete strength.
The fly ash-carbon air entrainment problem is an on-going issue that has been of concern since fly ash was first used nearly 75 years ago. Over the past ten years, these issues have been further exacerbated by regulations on environmental emissions which impose combustion conditions yielding fly ash with higher carbon contents. This situation threatens to make an increasingly larger portion of the available fly ash materials unsuitable for use in concrete. Considering the economic impact of such a trend, it is imperative to develop practical corrective schemes which will allow the use, with minimal inconvenience, of fly ash with high carbon contents (e.g., up to 10 wt %) in air-entrained concrete.
Air entrainment in fly ash-concrete may become yet more complicated by pending regulations that will require utilities to reduce current Hg emissions by 70-90%. One of the demonstrated technologies for achieving the Hg reduction is the injection of activated carbon into the flue gas stream after combustion so that volatile Hg is condensed on the high surface area carbon particles and discarded with the fly ash. Current practices are designed such that the added activated carbon is generally less than 1% by mass of the fly ash, but preliminary testing indicates this is disastrous when using the modified fly ash in air-entrained concrete.
The origin of air entrainment problems in fly ash concrete, and potential approaches to their solution, have been the subject of numerous investigations. Most of these investigations focussed on the ‘physical’ elimination of the carbon by either combustion processes, froth floatation, or electrostatic separation. To date, the proposed fly ash treatment approaches have found limited application due to their inherent limitations (e.g., separation techniques have limited efficiency in low carbon fly ash; secondary combustion processes are most suitable for very high carbon contents), or clue to their associated costs.
“Chemical” approaches have also been proposed to alleviate carbon-related problems in concrete air entrainment, for example through the development of alternative specialty surfactants for air entrainment agents such as polyoxyethylene-sorbitan oleate as an air entrainment agents (U.S. Pat. No. 4,453,978). Various other chemical additives or fly ash chemical treatments have been proposed, namely:                the addition of inorganic additives such as calcium oxide or magnesium oxide (U.S. Pat. No. 4,257,815); this invention prescribes the use of inorganic additives which may influence other properties of fresh mortars or concrete, for example, rate of slump loss and setting time;        the addition of C8 fatty acid salts (U.S. Pat. No. 5,110,362); the octanoate salt is itself a surfactant, and it is said to “stabilize the entrained air and lower the rate of air loss” (Claim 1 of U.S. Pat. No. 5,110,362);        the use of a mixture of high-polymer protein, polyvinyl alcohol and soap gel (U.S. Pat. No. 5,654,352); this discloses the use of protein and polyvinyl alcohol, and optionally a colloid (for example, bentonite) to formulate air entrainment admixtures;        treatment with ozone (U.S. Pat. No. 6,136,089); the ozone oxidizes fly ash-carbon, reducing its absorption capacity for surfactants and thus making the fly ash more suitable for use in air entrained systems.        
While each of the proposed solution may have potential merit, none has found significant acceptance in the industry, either because of their complexity and cost, or because of their limited performance in actual use. For example, a clear limitation to the addition of a second surfactant (e.g., C8 fatty acid salt), to compensate for the adsorption of the air entrainment agents surfactant, simply shifts the problem to controlling air content with a combination of surfactants instead of a single one. The problem of under- or over-dosage of a surfactant mixture is then the same as the problem discussed above with conventional air entrainment agents.
Hence, no practical solution currently exists which could efficiently relieve air entrainment problems for a wide variety of fly ash materials, in ready mix facilities or in the field.